A growing number of constructions is built at sea. A typical example concerns an offshore wind turbine, which typically comprises a gondola or nacelle, supported on a mast that can be more than 100 meters high. The mast is usually assembled from a number of mast segments that are placed successively on each other. The nacelle acts as housing for the required electrical equipment. The wind turbine is further provided with a rotor, which comprises a hub, onto which a plurality of blades is connected. The hub forms the connection between the rotor blades of the wind turbine and the nacelle. A generator in the nacelle transforms the rotary movement of the hub into electrical energy, and delivers this energy to the electrical grid network on the shore.
The rotor blades are large structures with lengths that may exceed 40 meters and more. The blade geometry is of course adapted to catch wind energy easily and efficiently. This makes assembly at sea, as well as transport of such blades from a shore location to an offshore wind energy park particularly difficult.
A known method for assembling a wind turbine at sea comprises connecting a plurality of blades to a hub on shore, and transporting the thus obtained rotor in assembled state, optionally together with mast segments and using a suitable vessel to an off shore jack-up platform in close proximity to the anticipated wind turbine location. A rotor is assembled on shore since it is considered too difficult to perform this operation at sea on a relatively small platform. Moreover, wind turbine and in particular rotor blade structures are lightly build, preferably from reinforced plastics, and are for this reason extremely fragile and vulnerable to damage. Assembling a rotor at sea would undesirably increase the risk for damage.
A crane provided on the jack-up platform transfers the mast segments and rotor from the transport vessel and manipulates them in order to assemble the wind turbine in close proximity to the platform. A known method takes each mast segment and places it on top of a previously positioned mast segment until the wind turbine mast is completed. After having added the nacelle on top of the mast, the crane present on the platform manipulates the assembled rotor to bring it into position and connect it to the nacelle.
The known method is time-consuming and uses a lot of space on the platform. Moreover, manipulating a rotor with a crane demands good coordination and skill, a lack of which may invoke danger and damage to the rotor.
It is an object of the present invention to provide a method of assembling a wind turbine at sea. It is another object of the present invention to provide a device and method for assembling a rotor of a wind turbine at sea. A further object of the invention is to provide an assembly of an offshore platform and a device for assembling a rotor of a wind turbine at sea, connected thereto.
These and other objects are achieved by a device for assembling a rotor of a wind turbine at sea, the device comprising a support structure that is provided with means for rotating the hub of a rotor around the hubs longitudinal axis, the support structure being adapted to be connected to an offshore platform such that the rotating means may be positioned outside the periphery of the platform. The device according to the invention is particularly suitable for efficiently assembling a rotor of a wind turbine at sea. By among others providing a support structure with means adapted to rotate the hub of a rotor, a rotor can be assembled at sea in a reliable manner whereby the risk for damage to the rotor is reduced. Now that a device is provided that allows to efficiently and reliably assemble a rotor at sea, the rotor does not need to be transported from an onshore location to the offshore location in assembled state, which significantly aids, and improves safety during such transport.
A hub of a rotor generally comprises a circumferential body of revolution with an axis of revolution corresponding to the longitudinal axis of the hub. A number of wind turbine blades is attached to the circumference of the hub to obtain a rotor. The blades extend outwardly and about perpendicular to the longitudinal axis of the hub.
The invention further relates to a method for assembling a rotor of a wind turbine at sea. The method comprises providing an assembly of a device according to the invention and an offshore platform, providing a plurality of blades and a hub that together define a rotor in unassembled state, attaching the hub to the rotating means of the support structure to obtain a first position of the hub, connecting a blade to the hub in said first position, rotating the hub around the hubs longitudinal axis to a second position, and connecting a second blade to the hub in said second position.
In another aspect of the invention, a method is provided wherein the sequence of rotating the hub from a first to a second position and connecting a blade to the hub in said second position is repeated until all blades have been connected to the hub, and the hub is released from the rotating means.
In yet another aspect of the invention, a method is provided wherein the hub is rotated from the first position to the second position around a substantially vertical axis. In this preferred embodiment, the hub is positioned in an upstanding position with its longitudinal axis substantially coinciding with the vertical direction, i.e. the direction perpendicular to the plane of the work deck of the platform. A blade that has been attached to the hub in a first position therefore will rotate in a substantially horizontal plane when rotating the hub to the second position. The first and second positions preferably coincide with rotational positions of the hub in which a blade is easily connected to the hub. Such positions preferably coincide with rotational positions of the hub in which attachment locations for the blades on the hub are readily accessible from the work deck of the platform. Such positions preferably include positions in which a to be assembled blade extends about parallel to an edge of the platform. Such a position allows to manipulate said blade from the platform in a well controlled manner, thereby avoiding damage to the blade.